Wireless acoustic communications method and apparatus

ABSTRACT

A method and apparatus for generating, propagating, encoding, decoding, modulating and detecting acoustic signals in a well borehole that comprises a signal generator ( 10 ) located at a first location for generating an acoustic source signal; a medium for the propagation the acoustic source signal to a second location and the propagation of the reflections and partial reflections of acoustic signals as it propagates through the medium; an apparatus to remove a detectable amount of energy from propagated acoustic signals; a controllable signal modulator ( 11 ) for attenuating the transmitted acoustic source signal; and a receiver ( 12 ) for receiving the modulated acoustic signals reflected, partially reflected or both by at least one reflector respectively located at one or more other locations.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a national phase entry under 35 U.S.C. §371 of International Application No. PCT/GB2013/050004 filed Jan. 4, 2013, published in English, which claims priority from Great Britain Patent Application No. 1200093.1 filed Jan. 5, 2012, all of which are hereby incorporated herein by reference.

This invention relates to a method and apparatus for transmitting information in a well borehole.

In general, a borehole comprises a jointed steel pipe called the production pipe that carries oil/gas from distances of several kilometres from the reservoir to the surface. Around the production pipe is an outer annulus that is normally filled with salt water or some other fluid. The outer annulus is contained by a casing, which is cemented in. The end of the production pipe is held in place by a packer. A number of sensors to measure temperature, pressure and other well parameters are placed along the borehole. Communicating with these sensors and other devices placed down a well borehole is a long standing problem for the oil and gas industry.

A variety of communication methods already exist, including electromagnetic, acoustic and wired. Wired solutions suffer from issues with the reliability and maintainability of the connection between pipe joints, and it is normally not possible to retrofit wired solutions. Generally, wireless solutions have the primary disadvantage of having to generate communication signals at every active point (i.e. node), which for down-hole components implies the availability of a local source of energy powerful enough to support the transmission of signals over the required communication distance. As a consequence, the majority of wireless solutions require batteries which are undesirable at the high temperatures typically found down-hole and having limited lifetimes requiring a need for well intervention to replace the batteries at expense to the operator of the well. Most methods also utilise a number of repeaters to send a signal, which further increases the maintenance cost. Data rates of such communication methods do not necessarily need to be high, as in certain situations even low data rates that return some information are useful, which helps to reduce battery consumption. Nevertheless higher data rates are always desirable.

This invention is mainly but not exclusively concerned with acoustic communication methods which use the production pipe as a communication channel and low energy consumption acoustic modulator apparatus that can be fitted or retrofitted to the production pipe. Communication methods using acoustic approaches propose the usage of the production pipe, the production fluid or the outer annulus fluid as a transmission medium. These methods are practical from an energy point of view due to the low acoustic attenuation of longitudinal and quasi-longitudinal waves in steel pipes, even over distances of several kilometres, and the low energy needed for attenuating those waves. This invention therefore reduces the number of repeaters needed to achieve long distance communications, potentially eliminating the need for repeaters altogether. Other type of pipes or borehole structures with good acoustic propagation properties would be equally suitable for the methods and apparatus covered by this invention, even if they are made of different materials or have different functionalities in the well.

An acoustic signal propagates through the bore-hole structure, tubing, fluid or casing in two directions, downward and upwards. For horizontal completions the terms downwards and upwards are to mean propagation towards the end of the well and propagation towards the top of the well respectively. Because of their different physical dimensions, the joints of jointed pipe present acoustic impedance discontinuities to the propagation of acoustic signals through the jointed pipe and will cause an acoustic signal to suffer many partial reflections at the joints. All these reflections contribute to the propagation of the acoustic signal in both directions and define the acoustic characteristics of jointed pipe structure.

The qualitative behaviour of this acoustic structure has been recognised by Barnes and Kirkwood in “Passbands for Transmission in an idealized drill string”, J. Acoustic Soc. Am. Volume 51, Issue 5B, pp. 1606-1608 (1972). Typically, transmission over long distances is supported within only a small number of narrow frequency pass bands in the acoustic spectrum. Those frequency bands are the best candidate communication channels for such a structure. The acoustic signals propagated on those bands include the transmitter generated signal and its partial joint reflections. Because the dimensions for joints in a typical production pipe are very short in comparison to the wavelength of sound, there is no easy way to distinguish between the transmitter signal and its reflections after a short period of time and the terms transmitter signal, propagated signal and reflected signals refer to different aspect of the same signal as it propagates through the borehole and are used to help clarify some aspects of the invention.

U.S. Pat. No. 4,293,936 discloses an acoustic telemetry method for transmitting signals over a string of standard drill pipe positioned in a borehole during drilling operations. It makes use of knowledge of the pass bands to ensure optimal transmission. The method has repeaters that repeat only at certain frequencies.

U.S. Pat. No. 0,086,336 discloses a semi passive two way borehole communication method whereby the signal is only generated at the top of the well, and is in some manner selectively reflected back up the well by a controllable reflector. The transmission medium is either considered to be the production fluid or the production pipe. The reflector is detailed as a mechanical device (a two position diaphragm reflector or a two position internal volume control Helmholtz resonator, both of which operate by changing the geometry of reflecting structure).

Acoustic communication concepts that are in existence include those that operate during drilling, and those that are permanently fixed down the hole. Both require the use of repeaters.

A difference to the requirements of communication while drilling is data rate. During drilling, a high data rate is necessary to react quickly to changes. During down-hole operations, the data rates can be lower as the general interest is in long term monitoring of the well condition.

The present invention seeks to address the above identified problems in bore-hole communications by utilising a low power device at the down-hole site and a transmission method that identifies and utilises low propagation attenuation acoustic signals. Depending on the borehole length and build, this invention could eliminate the need for batteries down-hole, maximise the life of any batteries and reduce the number of devices deployed down the well.

According to the present invention there is provided a communication system for a well borehole, the system comprising:

a controllable signal modulator for damping an acoustic signal in the well thereby removing a detectable amount of energy from the acoustic signal; and

a receiver for receiving the damped acoustic signal and detecting the amount of removed energy.

A corresponding method is also provided.

The acoustic signal may be provided by a transmitter. The transmitter and receivers may be positioned in the same location. The modulator may be arranged to operate with preference to one direction of acoustic propagation, causing different attenuation to downward signals than to upward signal or vice versa. The acoustic signal generator, the modulator and the receivers or any combination of them may be positioned in the same location. In another embodiment, a plurality of transmitter, modulator and receivers could be used over the same bore-hole. In another embodiment, a plurality of transmitter, modulator and receivers could be used on the same well where the tubing has been split into more than one branch.

The present solution is semi passive in that there is no need to generate an acoustic signal down within the well, reducing the amount of energy required by communication devices down-hole. Instead, the signal can be generated at the first location, for example at the well head (top of the borehole) where relatively easy access to power is available. This signal is then modulated before being reflected back up to the well head detector.

The present invention utilizes long propagation acoustic modes present in the bore-hole structure. In one embodiment, a longitudinal or quasi-longitudinal acoustic wave will be generated (along with other wave modes, which are not of interest due to their increased acoustic attenuation) at the well head. The signal will then propagate down the well to the proposed wireless acoustic modulator. This modulator will affect the signal by means of attenuation, such that energy is removed from the acoustic wave. The modulator will be able to appropriately modulate the acoustic wave to enable communication. A percentage of the signal is reflected from the device, the terminus of the well (the packer), and each individual pipe joint such that some energy will propagate back up towards the well head. At the well head, an acoustic receiver will detect the acoustic wave and determine, over a selected integration time, that the modulator device was either ‘on’ or ‘off’ (or potentially other states in between) by examining the characteristics of the returned acoustic wave. When the modulator device is on the amount of acoustic energy that can be sensed at another point of the bore-hole, like in the receiver, is different than when the modulator is off. For modulators that support more than two states, the amount of energy sensed will have more than two levels. The detectable change in energy at the receiver as the modulator changes state allows bits of information to be transmitted between modulator and receivers.

The apparatus may be arranged to provide two way communication (i.e. from the surface of the well to a location along the tubing and vice versa) by modulating the time between the transmission pulses, and introducing an acoustic sensor down-hole to detect the time change and thereby retrieve a message. Alternatively, the current generated within the acoustic modulator could be used as a detector.

This solution improves on existing acoustic communication concepts by acoustically modulating the signal using an innovative electromagnetic device, as opposed to detecting the signal at the bottom of the well and retransmitting or by just reflecting the signal. Some of the advantages of this invention over prior art are the following. Firstly, there are low power requirements for down hole communication nodes, the acoustic modulator requires only a low power switch(es) to effect the modulation that allows a message to be transmitted between communication nodes.

Secondly, there are no high power electromechanical parts: The solution uses acoustic-electromagnetic effects, and requires no high power mechanically active parts down-hole. Furthermore, there are a reduced number of down-hole components, reducing the cost of installation and any maintenance as opposed to solutions which require acoustic repeaters to be installed along the well, in many circumstances, one down-hole device might be sufficient.

Examples of the present invention will now be described with reference to the accompanying drawings, in which:

FIG. 1 shows an example of a communication apparatus according to the invention;

FIG. 2 is a cross sectional view of an example electromagnetic device for an acoustic modulator for the apparatus of the invention;

FIG. 3 is a perspective view of the example electromagnetic device of FIG. 2;

FIG. 4 is an example cross section of the electromagnetic device of FIGS. 2 and 3;

FIG. 5 is an overall view of an embodiment of the invention showing potential specific locations for certain components;

FIG. 6 shows some of the elements of the method of the invention for an up-hole communication;

FIG. 7 is a block diagram showing the main steps of an up-hole communication enabled by the invention;

FIG. 8 shows some of the elements of the method of the invention for a down-hole communication; and

FIG. 9 is a block diagram showing the main steps of a down-hole communication enabled by the invention.

Referring to FIG. 1, an apparatus according to the invention comprises the following elements. An acoustic transmitter 10 positioned at location A in FIG. 1; an acoustic modulator 11 positioned at location B in FIG. 1, and

an acoustic receiver 12 positioned at location A in FIG. 1.

The acoustic transmitter 10 is arranged to broadcast a longitudinal or quasi-longitudinal acoustic wave. The operational frequencies of this acoustic wave are selected to suit the acoustic characteristic of the pipe 13. The physical build of the transmitter and the driving waveforms used to excite it are designed to maximize the efficient generation of acoustic propagating modes with long distance propagation in the borehole. The acoustic transmitter is arranged to provide the following functionality:

a. the generation of acoustic signals to enable downhole communication nodes to transmit information using acoustic modulators;

b. the generation of acoustic signals to enable top of the well equipment to communicate with downhole devices;

c. the generation of acoustic signals to identify most favourable propagation frequencies and waveforms for the borehole; and

d. the generation of acoustic signals to detect and track changes in the acoustic characteristics of the borehole.

The acoustic transmitter 10 can be built using different electro-acoustic transduction principles, such as air coil speakers, piezoelectric, magnetostriction, electromechanical shaker and other electro-acoustic technologies. It will be appreciated that the acoustic transmitter can be built with one or more than one transducer, with one or more than one transducer technology, with one or more than one transducer in one location and with more than one transducer in more than one physical location.

An acoustic transmitter encoder 14 is provided. The acoustic transmitter encoder 14 maps the logical information to transmit into appropriate signals used to drive the acoustic transmitter. As an example, an acoustic transmitter encoder 14 may map a bit of information to be transmitted by a four element acoustic transmitter array into four different waveforms that drive four different transducers at four different locations. The acoustic transmitter encoder may perform message encoding to facilitate the error detection and error correction at the acoustic receiver 12.

An acoustic transmitter controller 15 is also provided. The acoustic transmitter controller 15 operates the acoustic transmitter 11 and encoder 14 and is an interface to other equipment like a controlling computer system (not shown) that might be used to generate the information to be transmitted.

The modulation of an acoustic wave is by means of an acoustic-electromagnetic device 11. An example is depicted in FIGS. 2 to 4. The device has an outer series of conducting coils 1. These coils 1 can either be connected together and switched as one, or disconnected and switched independently. The modulator length can be varied to optimize its performance. Elastically coupled to the inside of the coils 1 is a series of ring (or cylindrical) magnets 2 with the North and South poles along the main axis of the cylindrical production pipe 4. The coils 1 are positioned such that they experience the maximum radial flux from the magnets 2. The magnets will be separated by spacers 3. The modulator is fixed by a device fixing 5 to the inner walls of the device (for simple retrofitting) with optimum acoustic coupling. The device can be coupled either along the entire length of the device, 5 or using other conventional methods for retrofitting devices inside a well bore that provide suitable acoustic coupling. The modulator can be configured either to allow production fluid flow through the middle of the device, or around its perimeter. A low power FET switch or other device is used to short and open the coils 1 by the sensor or device that is attached to it. The coils may be connected to ensure the direction of the force from the coil/flux linkage is maximised.

The acoustic modulator device 11 makes use of damping to remove energy from the incoming acoustic wave. This damping of the acoustic signal can be induced mechanically, electromagnetically or by a combination of both of them.

In the case of mechanical damping, this is accomplished by coupling the acoustic wave to a mechanical oscillator, therefore transferring a portion of the acoustic energy from the pipe to the mechanical oscillator. The energy transferred to the mechanical oscillator is dissipated in a damper or transferred to a mechanical oscillation mode that is not compatible with linear or quasi-linear acoustic propagation through the pipe or by a combination of both methods. In the first implementation, the mechanical oscillator is selectively switched between two or more damping levels to modulate the acoustic energy levels being propagated though the tubing. In the second implementation, the mechanical oscillator is selectively switched between two or more oscillation modes incompatible with acoustic propagation through the pipe, subtracting different amounts of energy from the pipe propagation mode and in this way modulating the acoustic energy levels being propagated though the tubing. A combination of both methods can be combined to modulate the acoustic energy levels being propagated though the tubing.

In the case of electromagnetic damping, this is accomplished by ensuring that the acoustic wave transfers some of its energy to conducting coils which will then move relative to a series of magnets. The relative motion between the conducting coils and the magnets will cause a current to flow in the coils if electrically shorted, which in turn sets up an opposing magnetic field which will cause a retarding force, i.e. a force that is against the direction of motion. it is this set of forces that couples the acoustic wave propagating through the pipe and coils to the magnets. If the coils themselves are not electrically shorted, then a current will not be able to flow, disabling the coupling between the acoustic waves in the pipe and the magnets. Thus, by switching the coils between open and closed, the acoustic signal can be effectively modulated.

In the case of combined mechanical and electromagnetic damping, this is accomplished attaching the magnets to a mechanical oscillator and coupling the acoustic wave to the mechanical oscillator by means of electrically conductive coils attached to the pipe. The controlled damping of energy and can be accomplishing in a number of ways:

1. by adding a controllable mechanical damper to the mechanical oscillator;

2. by adding a mechanical damper to the mechanical oscillator and varying the strength of the coupling between the acoustic wave and the mechanical oscillator;

3. by adding a mechanical damper to the mechanical oscillator and varying the strength of the coupling between the acoustic wave and the mechanical oscillator by selecting the number of coils switched to open and close circuit position;

4. by adding a mechanical damper to the mechanical oscillator and varying the strength of the coupling between the acoustic wave and the mechanical oscillator by selecting the electrical resistivity of coils by means of a controllable resistance device;

5. by adding a mechanical damper to the mechanical oscillator and varying the strength of the coupling between the acoustic wave and the mechanical oscillator by selecting the electrical resistivity of coils by means of a controllable resistance device, where the controllable resistance device is a transistor, or a vacuum valve, or a variable resistor, or a switch with more than one pole attached to fixed resistors, or a combination of the pervious elements that provides a means to change the level of resistivity of coil circuit, or a combination of electrical circuit elements that provides a means to change the level of resistivity of the coil circuit;

6. any of the methods described above where a mechanical oscillator mode not compatible with propagation is used instead of the mechanical damper.

An acoustic receiver 12 is provided. The acoustic receiver 12 operation is based on an electro-acoustic transduction principle with the sensor configured to convert acoustic signals into electrical signals, like an air coil microphone. Other implementations could be used based on other electro-acoustic technologies like piezoelectric, magnetostriction, electromechanical shaker and others. The acoustic receiver can be built with one or more than one transducer, with one or more than one transducer technology, with one or more than one transducer in one location and with more than one transducer in more than one physical location.

An acoustic receiver decoder 16 maps the acoustic receiver signal into information that can be stored digitally. A number of analogue electrical signals are generated by the acoustic receiver 12. These signals can be filtered and processed in the analogue domain and converted to digital signals in an analogue to digital converter. The signal can them be further filtered and processed in the digital domain to extract the communication information. The acoustic receiver decoder unit may perform message decoding, error detection and error correction functions.

The apparatus of the invention and its method of operation can be optimised on a well by well basis to ensure that the signal transmitted at the top of the well is tuned to the transmission medium. As the transmission medium might change over the life of the well, this optimisation could take place periodically.

Adaptations to the invention include the following.

Different coil on/off combinations. When the coils 1 are grouped with multiple switches for different sets, then with a suitably sensitive detector and a strong enough input signal at the top of the well, different groups and patterns can be switched such that the waves are attenuated at different levels. This allows transmission of more than one bit per pulse depending on the level of attenuation.

Mounting acoustic modulator outside of the pipe. When the acoustic modulator is not for a retrofit solution it could be built in a special joint of pipe with the modulator onto the outside of the production pipe. This implementation does not interfere with the flow inside the production pipe.

Transmission on more than one frequency band. When two or more different pass frequencies can be found to propagate for long enough distances, it is possible to transmit acoustic pulses at the found different frequencies simultaneous or close in time. This enables the possibility of simultaneous or almost simultaneous transmission to and from two different communication nodes with acoustic modulators optimized to operate and different frequency bands.

Continuous carrier wave. The transmitted wave could be a continuous source rather than a pulse. The device would not then have to wait for the travel time from the top of the well to the device. The modulator could then switch faster, and the detector method at the wellhead would have to distinguish between overlapping signals to deduce the on and off states. This would allow a higher data rate.

Use naturally occurring noise. The transmitted wave could be naturally occurring noise in the borehole. The modulator attenuates the noise propagating through the borehole and the receiver can detect the changes in noise energy levels induced by the modulator.

Referring to FIG. 5, it can be seen how a combined transmitter and receiver unit 10, 12 can be attached to tubing 13 via a tubing hanger 20 at the head of a well. The modulator 11 in this example is positioned within the tubing 13 at a location down well.

Referring to FIG. 6, the main steps of up-hole communication are shown, in which a parameter is detected by sensors, data from the sensors encoded and modulated and provided to an acoustic transducer for transmission. These are received by an acoustic transducer, demodulated and decoded for processing.

FIG. 7 shows a little more detail of these key steps, with the information being placed into a frame structure correction encoded, mapped in a logical to physical format and then provided to a modulator transducer for passage through the acoustic channel, in this example in the form of pipe 13. This is received by a demodulator transducer and followed by a physical to logical mapping, error correction decoding, the provision of frame structure and then an output of information.

FIG. 8 shows down hole communication in which data to be encoded is provided to a modulator which applies it to an acoustic carrier for transmission down the well via pipe 13. The acoustic transducer at the other end receives the acoustic signal, following which it is demodulated and decoded.

FIG. 9 shows the steps in more detail. 

1. A communication system for a well borehole, the system comprising: a controllable signal modulator for damping an acoustic signal in the well thereby removing a detectable amount of energy from the acoustic signal; a receiver for receiving the damped acoustic signal and detecting the amount of removed energy; and an acoustic propagation medium for propagating the acoustic signal, modulated by a controllable signal modulator located at a first location, to a second location; wherein the receiver is arranged to detect the acoustic signal, and wherein the controllable signal modulator comprises a plurality of conductive coils moveable relative to a plurality of magnets.
 2. A system according to claim 1, the system further comprising: a signal generator located at a third location for generating an acoustic source signal.
 3. A system according to claim 1, wherein the acoustic propagation medium comprises a pipe disposed in the borehole.
 4. A system according to claim 2, wherein the third location is a location at the top of the borehole.
 5. A system according to any of claim 1, wherein the first location is within the borehole.
 6. A system according to claim 5, wherein the first location is at the end of a pipe disposed in the borehole or at a pipe joint.
 7. A system according to claim 1, wherein the receiver is located at the first location. 8.-9. (canceled)
 10. A system according to claim 1, wherein the conductive coils and the magnets are coupled to a pipe.
 11. A system according to claim 9, claim 7, wherein the magnets have poles arranged symmetrically along the axis of the borehole.
 12. A system according to claim 1, wherein the magnets are aligned with the conductive coils such that, in use, magnetic flux through the coil is optimised.
 13. A system according to claim 1, wherein the conductive coils are selectively connected to vary the amount of damping.
 14. A system according to claim 1, wherein the controllable signal modulator is located within a pipe.
 15. A system according to claim 1, wherein the acoustic signal is transmitted at two or more frequencies.
 16. A system according to claim 1, wherein the acoustic signal is continuous.
 17. A method of communicating in a well borehole, the method comprising the steps of: damping an acoustic signal in the borehole thereby removing a detectable amount of energy from the propagated acoustic signal in the well; receiving the damped acoustic signal to detect the amount of removed energy; and propagating the acoustic signal from a first location, wherein the signal modulator modulates the acoustic signal, to a second location, wherein the receiver is arranged to detect the acoustic signal, and wherein the controllable signal modulator comprises a plurality of conductive coils moveable relative to a plurality of magnets.
 18. A method according to claim 17, the method further comprising the steps of: generating at a third location an acoustic source signal.
 19. A method according to claim 17, wherein the acoustic propagation medium comprises a pipe disposed in the borehole.
 20. A method according to claim 18, wherein the third location is a location at the top of the borehole.
 21. A method according to claim 17, wherein the first location is within the borehole.
 22. A method according to claim 17, wherein the first location is at the end of a pipe disposed in the borehole or at a pipe joint.
 23. A method according to claim 17, wherein the receiver is located at the first location. 24.-25. (canceled)
 26. A method according to claim 17, wherein the conductive coils and the magnets are coupled to a pipe. 27.-32. (canceled) 